Monoamine reuptake inhibitors elevate extracellular levels of serotonin (5-HT), norepinephrine (NE) and/or dopamine (DA) in the brain by binding to one or more of the transporters responsible for reuptake, namely the serotonin transporter (SERT), the norepinephrine transporter (NET) and the dopamine transporter (DAT), thereby blocking reuptake of the neurotransmitter(s) from the synaptic cleft. Monoamine reuptake inhibitors are an established drug class that has proven utility for the treatment of a number of CNS disorders especially major depressive disorder (MDD).
Since the introduction of tricylic antidepressants (TCAs) almost 50 years ago, monoamine reuptake inhibitors with greatly improved safety profiles have significantly enhanced the treatment of depression. Although TCAs are very effective antidepressants, cardiovascular, anticholinergic and sedative side effects are common due to the interaction of TCAs with muscarinic, histaminic and adrenergic receptors. The revolutionary introduction of selective serotonin reuptake inhibitors (SSRIs) in the 1980s allowed a much larger patient population to be treated because of the highly improved safety profile. Over the past decades, inhibitors that selectively block the reuptake of NE or DA, or two of the three neurotransmitters simultaneously, have become available for the treatment of CNS disorders including depression, anxiety, obsessive compulsive disorder (OCD), attention deficit hyperactivity disorder (ADHD), pain and urinary incontinence. Two representative recent reviews (Liu and Molino, Annual Reports in Medicinal Chemistry, 42:13 (2007); Walter, Drug Dev. Res., 65:97 (2005)) on monoamine reuptake inhibitors summarize the history and recent development in the monoamine reuptake inhibitor area.
Currently, the major effort in the field of monoamine reuptake inhibitors is focused on improving antidepressant efficacy since 30-40% of patients do not respond to treatment with currently available antidepressants. An additional major objective is to enhance the onset of action. Current antidepressants typically require 2-6 weeks of treatment before clinical efficacy is seen. Clinical trials exploring augmentation strategies, in which a DA reuptake inhibitor or a dual NE/DA reuptake inhibitor is combined with an SSRI, have resulted in improved efficacy in depressed patients refractory to SSRI treatment alone (Patkar et. al, J. Clin. Psychopharmacol., 26:653 (2006); Zisook et al, Biol. Psychiat., 59:203 (2006)). The improved results from clinical trials such as these serve to justify the considerable focus on the development of inhibitors that simultaneously block the reuptake of 5-HT, NE and DA. Because of the continued need for better drugs to treat depression and the opportunities for new clinical indications, efforts to discover novel monoamine reuptake inhibitors continue unabated.
Methylphenidate, currently used for the treatment of attention deficit-hyperactivity disorder, is known to be selective for inhibition of the DAT. Also, U.S. Pat. No. 5,444,070 discloses selective inhibitors of dopamine reuptake as treatments for Parkinson's disease, drug addiction or abuse including cocaine and amphetamines.
Selective norepinephrine reuptake inhibitors (NARI) have also been disclosed. U.S. Pat. No. 6,352,986 describes methods of treating attention deficit-hyperactivity disorder (ADHD), addictive disorders, and psychoactive substance use disorders with Reboxetine. Also, Atomoxetine (STRATTERA®) is currently marketed as a selective NET reuptake inhibitor for ADHD.
The use of selective serotonin reuptake inhibitors (SSRI) has been shown to be effective in treating depressive disorders. Sertraline, citalopram, escitalopram, paroxetine, fluoxetine and fluvoxamine are well known examples of SSRIs used to treat disorders such as depression, obsessive compulsive disorder, and panic attacks. There are several known difficulties with the SSRI class of therapeutics, including the slow onset of action, unwanted side effects, and the existence of a significant subset of the population that is not responsive to SSRI therapy. Recent effort in the clinical development of new SSRIs has focused on the treatment of premature ejaculation (PE) by taking advantage of the ejaculation-delaying side effects of SSRIs. Although SSRIs have been prescribed off-label to treat this condition, an SSRI with rapid onset of action and rapid clearance could be preferred for on-demand treatment of PE. Dapoxetine (LY210448), an SSRI structurally related to fluoxetine with a shorter half-life, was reported to be an effective and generally well tolerated treatment for men with moderate-to-severe PE in clinical trials (Feret, Formulary, 40:227 (2005); Pryor et al, Lancet, 368:929 (2006)).
Selective inhibitors of DAT, NET, and SERT reuptake may also be co-administered with each other or with other drugs. U.S. Pat. No. 5,532,244 discloses the use of serotonin reuptake inhibitors in combination with a serotonin 1A antagonist for the treatment of obsessive-compulsive disorder, depression, and obesity. The use of a serotonin or norepinephrine reuptake inhibitor in combination with a neurokinin-1 receptor antagonist has been disclosed in U.S. Pat. No. 6,121,261 for the treatment of ADHD. U.S. Pat. No. 4,843,071 discloses the use of a norepinephrine reuptake inhibitor in combination with a norepinephrine precursor in the treatment of obesity, drug abuse, or narcolepsy. U.S. Pat. No. 6,596,741 discloses the use of a NE, DA, or 5-HT inhibitor with either a neurokinin-1 receptor antagonist or a serotonin-1A antagonist for the treatment of a wide variety of conditions.
Also advantageous is the use of compounds that inhibit one or more of the neurotransmitters at the same time. The antidepressant qualities of the dual NET and SERT reuptake inhibitor duloxetine is disclosed in European Patent No. EP 273658. Venlafaxine is disclosed in U.S. Pat. No. 4,535,186 as a reuptake inhibitor of both NE and 5-HT for the treatment of depressive disorders. U.S. Pat. No. 6,635,675 discloses the use of the dual NE and 5-HT reuptake inhibitor milnacipran for the treatment of chronic fatigue syndrome and fibromyalgia syndrome. In addition, dual NE and 5-HT reuptake inhibitors are also disclosed in U.S. Pat. No. 6,136,083 for the treatment of depression. It is also recognized that compounds which inhibit the reuptake of NE, DA, and 5-HT in varying ratios not specifically mentioned here would also be advantageous.
As the first SNRI drug approved, venlafaxine has become one of the first-line choices for depression and anxiety disorder. An active metabolite, desvenlafaxine, is also under clinical development for the treatment of major depressive disorders. Preclinical studies also indicate that desvenlafaxine may be effective in relieving vasomotor symptoms associated with menopause (e.g., hot flashes and night sweats) (Sorbera, et al, Drugs of Future., 31:304 (2006); Albertazzi, J. Br. Menopause Soc., 12:7 (2006)). Desvenlafaxine is reported to be in clinical development for the treatment of fibromyalgia and neuropathic pain, as well as vasomotor symptoms associated with menopause.
In addition to treating major depressive disorder, duloxetine was approved as the first agent for the treatment of painful diabetic neuropathy in the U.S. It also has been used for stress urinary incontinence in women in Europe. In 2007, duloxetine was approved for the treatment of generalized anxiety disorder in the U.S. Most recently, it was approved by the FDA for the management of fibromyalgia.
Milnacipran is currently available for use as an antidepressant in several countries outside the U.S. It is also under clinical development to assess its potential role in the treatment of fibromyalgia syndrome.
After more than a decade of use, bupropion, is considered a safe and effective antidepressant, suitable for use as first-line treatment. In addition, it is approved for smoking cessation and seasonal affective disorder. It is also prescribed off-label to treat the sexual dysfunction induced by SSRIs. Bupropion is often referred to as an atypical antidepressant. It has much lower affinity for the monoamine transporters compared with other monoamine reuptake inhibitors. The mechanism of action of bupropion is still uncertain but may be related to inhibition of dopamine and norepinephrine reuptake transporters as a result of active metabolites. In a recently reported clinical trial, bupropion extended release (XL) had a sexual tolerability profile significantly better than that of escitalopram with similar remission rates and Hospital Anxiety and Depression (HAD) total scores in patients with major despressive disorder (Clayton et al. J. Clin. Psychiatry, 67:736 (2006)).
Treating illnesses by inhibiting the reuptake of all three of the monoamines either through combination therapy or “triple inhibitors” may have clinical benefit as well. Triple inhibitors are considered to be the next generation antidepressant (Liang and Richelson, Primary Psychiatry, 15(4):50 (2008)). Rationale for inclusion of a dopamine enhancing component in anti-depressant therapy includes observed deficits in dopaminergic function, the success of combination therapy with dopamine agonists and traditional anti-depressants, and an increased sensitivity in dopamine receptors due to chronic anti-depressant administration (Skolnick et al., Life Sciences, 73:3175-3179 (2003)). Combination therapy with an SSRI and a noradrenaline and dopamine reuptake inhibitor was shown to be more efficacious in patients with treatment-resistant depression (Lam et al, J. Clin. Psychiatry, 65(3):337-340 (2004)). Clinical studies using the combination of bupropion and an SSRI or SNRI have showed improved efficacy for the treatment of MDD in patients refractory to the treatment with SSRIs, SNRIs, or bupropion alone (Zisook et al, Biol. Psychiat., 59:203 (2006); Papkostas, Depression and Anxiety, 23:178-181 (2006); Trivedi et al, New Engl. J. Med., 354:1243 (2006)). Other studies using methylphenidate, both immediate release and extended release formula, have shown it to be effective as an augmenting agent in treatment-resistant depression (Patkar et al, J. Clin. Psychopharmacol., 26:653 (2006); Masand et al, Depression and Anxiety, 7:89 (1998)). In addition, the combination of bupropion-SR with either SSRIs or norepinephrine and dopamine reuptake inhibitors was found to induce less sexual dysfunction than monotherapy (Kennedy et al, J. Clin. Psychiatry, 63 (3):181-186 (2002)). As such, inhibitory activity against DA reuptake, in addition to NE and 5-HT reuptake, is expected to provide a more rapid onset of anti-depressant effect than other mixed inhibitors which are selective for NET and SERT over DAT. PCT International Publication Nos. WO 03/101453 and WO 97/30997 disclose a class of compounds which are active against all three monoamine transporters. Also, PCT International Patent Publication No. WO 03/049736 discloses a series of 4-substituted piperidines, each of which displays similar activity against DA, NE, and 5-HT transporters. Bicyclo[2.2.1]heptanes (Axford et al., Bioorg. Med. Chem. Lett., 13:3277-3280 (2003)) and azabicyclo[3.1.0]hexanes (Skolnick et al., Eur. J. Pharm., 461:99-104 (2003)) are also described as triple inhibitors of the three monoamine transporters. 1-(3,4-Dichlorophenyl)-3-azabicyclo[3.1.0]hexane has been shown to be efficacious in treating depression in clinical trials (Beer et al, J. Clin. Pharmacol., 44:1360-1367 (2004)). Current widely used anti-obesity drug sibutramine is believed to work through the inhibition of all three transporters DAT, SERT, and NET (Ryan, Pharmacotherapy of Obesity, 245-266 (2004)).
Recent drug approvals with SNRIs for treatment of fibromyalgia and diabetic neuropathy reinforce the utility of this drug class in the treatment of neuropathic pain. Other largely untapped areas which remain to be exploited with this drug class include sexual dysfunction, such as premature ejaculation, irritable bowel syndrome, obesity, neurodegenerative diseases such as Parkinson's disease, restless leg syndrome, and substance abuse and addiction.
Klumpp et al., Organic Letters 3:2781-2784 (2001) describes the synthesis of compound of formula I via acid catalyzed intramolecular cyclization. No biological activity of this compound is reported in the above-mentioned reference.

There is still a large need for compounds that block the reuptake of norepinephrine, dopamine, and serotonin and treat various neurological and psychological disorders.
The present invention is directed to achieving this objective.